branch: move 'current' check down to the presentation layer

branch: move 'current' check down to the presentation layer

We check if given ref is the current branch in print_ref_list(). Move
this check to print_ref_item() where it is checked right before
printing. This enables a smooth transition to using ref-filter APIs,
as we can later replace the current check while printing to just check
for FILTER_REFS_DETACHED instead.

Based-on-patch-by: Jeff King <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Matthieu Moy <redacted>
Signed-off-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>